1. Field of the Invention
The present invention relates to a sheet discharging apparatus and an image forming apparatus, particularly to the sheet discharging apparatus and image forming apparatus provided with a pressing member which presses sheets stacked in a sheet stacking portion from above.
2. Description of the Related Art
Conventionally, an image forming apparatus, such as a printer, a facsimile, a copying machine, and a multi function printer, which includes a sheet discharging apparatus. The sheet discharging apparatus discharges a sheet in which an image is formed to a sheet discharge portion and stacks the sheet on a sheet stacking portion provided below the sheet discharge portion.
In the image forming apparatus, when the image is formed in the sheet, a photosensitive drum whose surface is charged in a predetermined polarity is irradiated with light based on image information, thereby forming an electrostatic latent image on a surface of the photosensitive drum.
A development device supplies toner charged in the same polarity as the photosensitive drum to the photosensitive drum on which the electrostatic latent image is formed, and the toner is adsorbed to a portion of the photosensitive drum in which the charge does not exist, thereby developing the electrostatic latent image. Then a conveying portion including a sheet feeding roller and a conveying roller conveys a sheet to which the toner image is transferred to a nip portion between the photosensitive drum and a transfer roller. When the transfer roller applies charges having an opposite-polarity to the toner to the backside of the sheet, and the toner on the surface of the photosensitive drum is attracted onto the sheet.
Then the sheet to which the toner image is already transferred is conveyed to a nip portion of a fixing portion. The nip portion of the fixing portion is formed by a heating roller into which a heating device is incorporated and a pressure roller that pressure-contacts the heating roller. The fixing portion heats and pressurizes the sheet to fix the toner image to the sheet. The sheet to which the toner image is already fixed is discharged from a sheet discharge port to a sheet stacking portion by a pair of discharge rollers provided in the sheet discharging apparatus. In order to secure alignment performance, the sheet stacking portion is upwardly inclined toward a downstream side in a sheet discharge direction.
In the case where the sheet is heated by the fixing portion, because a difference in sheet shrinkage amount is generated between the heating roller side and the pressure roller side, a curl (warp, curvature) is generated in the sheet discharged to the sheet stacking portion. When the curl is generated, a sheet stacking property is deteriorated in the sheet stacking portion.
The curl tends to be prominently generated in a high-temperature, high-humidity environment when the sheet has a large amount of water absorption. Therefore, in a conventional sheet discharging apparatus, a pressing member is provided above the sheet discharge port in order to improve the sheet stacking property. The pressing member presses a neighborhood of a tailing end of the sheet stacked on the sheet stacking portion from above to suppress the curl of the sheet.
Conventionally, the sheet has a feature that the sheet is easily stretched and contracted in a direction parallel to a fiber array (paper making grain) direction. That is, the curl direction depends on the fiber array (long grain and short grain) direction of the sheet. The sheet having the paper making grain in the same direction as the sheet discharge direction has a curl shape (hereinafter referred to as long grain curl) in which both ends in a direction (hereinafter referred to as width direction) orthogonal to the sheet discharge direction are floated. On the other hand, the sheet having the paper making grain in the width direction has a curl shape (hereinafter referred to as short grain curl) in which a leading end and tailing end in the sheet discharge direction are floated. The sheet is cut such that a long side of the sheet is matched with the long grain direction in many cases.
Usually, the pressing member has a rotational support located above the pair of discharge rollers, and the pressing member is pressed against and lifted by the sheet so as not to obstruct the sheet discharge during discharging the sheet. In other cases, a lower end of the pressing member is located below the nip portion of the pair of discharge rollers in order that the sheet stacked on the sheet stacking portion is not lifted to the nip portion of the pair of discharge rollers.
In the conventional sheet discharging apparatus, when the sheet has the short grain curl, as illustrated in FIG. 18, the sheet end portion that is curled in the sheet discharge direction easily hides in a space between a pair of discharge rollers 200 and a pressing member 201. When a sheet S hides in the space, the pressing member 201 insufficiently exerts a function of pressing the sheet S, and the sheet S blocks a nip of the pair of discharge rollers 200. As a result, sometimes the sheet S blocking the nip of the pair of discharge rollers 200 is damaged, or paper jam is generated in a sheet discharged subsequent to the sheet S.
Conventionally, there is a sheet discharging apparatus in which a fully loaded state (predetermined maximum stacking height) of the sheets stacked on the sheet stacking portion is detected such that the sheet does not block the nip of the pair of discharge rollers, and there is a sheet discharging apparatus in which the pressing member is used to detect the fully loaded state of the sheets. However, when the sheet is curled in the sheet discharge direction, because the pressing member insufficiently exerts the function of pressing the sheet as described above, the fully loaded state of the sheets is hardly detected using the pressing member.
In order to the problem, various configurations are proposed in the conventional sheet discharging apparatus. For example, irregularities are provided in one of the pair of discharge rollers to impart rigidity to the sheet, thereby preventing the floating of the sheet tailing end. A method for early detecting the fully loaded state by downwardly stretching the pressing member and a method for curving the pressing member toward a stacking wall side without stretching the pressing member in order to detect the sheet near the stacking wall are also proposed.
Additionally, for example, Japanese Patent Application Laid-Open No. 2003-246535 proposes a method in which the fully loaded state of the sheets is detected while an upper end of the sheet stacking wall is projected toward the downstream side in the sheet discharge direction to hold the sheet swelling of the tailing end portion. Japanese Patent Application Laid-Open No. 9-48558 also proposes a configuration, in which turnable regulating member is projected from the stacking wall surface side to press the upstream end in the sheet discharge direction each time the sheet is stacked, whereby the breakage of the sheet stacked on the sheet stacking portion can be prevented while the sheet is discharged without catching the sheet.
In the recent image forming apparatus, particularly in the recent full-color laser printer, a demand for a glossy degree (gloss) of the image has been largely increased. However, in the conventional sheet discharging apparatus in which the irregularities are provided in one of the pair of discharge rollers in order to improve the sheet stacking property, the need for the glossy degree cannot be satisfied because uneven gloss or an irregularity trace is generated.
In the method for early detecting the fully loaded state by downwardly stretching the pressing member in order to improve the sheet stacking property, because the fully loaded state of the sheets that are not curled is also detected early, the method is not suitable to the case in which a stacking capacity is ensured in the restricted space. In the method for curving the pressing member toward the stacking wall side without stretching the pressing member in order to detect the sheet near the stacking wall, the discharged sheet is possibly wrapped around the pressing member when colliding with the pressing member.
In the configuration disclosed in Japanese Patent Application Laid-Open No. 2003-246535, because the upper end of the sheet stacking wall is projected toward the downstream side in the sheet discharge direction to hold the sheet swelling of the tailing end portion, the leading end portion of the pressed sheet is moved toward the downstream side in the sheet discharge direction. Therefore, in the case where the configuration disclosed in Japanese Patent Application Laid-Open No. 2003-246535 is applied to the in-apparatus discharge type image forming apparatus, the downstream end in the sheet discharge direction and the lower surface of the image reading apparatus are brought close to each other to collide easily with each other.
Additionally, because the tailing end portion of the sheet is pressed, when the sheet curled in the sheet discharge direction starts the collision with the lower surface of the image reading apparatus, the sheet has nowhere to go, and the physically stranded sheet cannot be prevented from colliding with the sheet. In the configuration disclosed in Japanese Patent Application Laid-Open No. 9-48558, because the turnable regulating member is provided, the control and configuration become complicated to increase cost.
The present invention relates to a sheet discharging apparatus and an image forming apparatus, in which even a curled sheet can properly be discharged to and stacked on a sheet stacking portion.